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Better throughput, wet print accuracy and cycle time

SMT paste printing challenges
Better throughput, wet print accuracy and cycle time

Better throughput, wet print accuracy and cycle time
The paste printer Yamaha YCP-10 delivers quality similar to flag-ship models, compatible with large circuit boards and a wide variety of stencil frames, its high-end features guarantee for optimum print accuracy and quality. Source: Yamaha
Current trends in the production of mobile consumer electronics and automotive products are driving the SMT stencil printing process to achieve higher throughput with correspondingly higher wet print accuracies. Process challenges are imposed by such as larger (panelized) PCB handling requirements, shrinking PCB/PCBA (printed circuit board assemblies) topographies with smaller components and tighter designs (fine pitch and alike), and the requirements of “broadband printing’.

Michael L. Martel and Ed Nauss, Yamaha Motor IM America

Printer suppliers are responding to these demanding issues with new and enhanced techniques that improve volume distribution and print definition, holding the correct print process window open. For example, more printers use side-clamping systems for more accurate process results and fewer volumetric variations across the board. Also, the ability to adjust squeegee angle has improved print consistency from edge to edge and increased the aperture fill on experimental materials, something that we are seeing more and more of as PCB assembly technology advances into new territories.
What are the challenges?
  • Need for higher throughput (sometimes referred to as speed)
  • Need for higher wet print accuracy
  • Larger PCBs/panelized PCBAs (warpage, handling, work area)
  • Shrinking PCB/PCBA topographies
  • Even finer features, fine-pitch components, etc.
  • Broadband printing, defined as a process that delivers the solder paste volume required for a PCB with both miniature and larger device patterns in parallel.
How are printer manufacturers responding?
    • Increasing printer accuracy
    • Raising printer throughput capability
    • Improving overall printing process efficiency
    • Growing PCB panels handling size
    • Advancing existing features
    • Developing machines with more advanced features.
Achieving optimum accuracy
  • Printing machine sales people toss the term accuracy around a lot, but what does it really mean? Are we referring to machine accuracy, as in quality of construction and tolerances, or performance? And if the latter, then what, specifically, do we mean?
  • Software improvements and enhancements can bolster accuracy
  • Tighter tolerances in printer machining and manufacture. Remember, when it comes to variations in tolerances, add them together for cumulative total accuracy or inaccuracy
  • Reduction in vibration through minimization of unnecessary motion of moving parts (gantries, etc.)
  • Less overall motion also means less machine wear, longer life operating within specs
  • Wet print accuracy – the only kind that really counts.
True printer accuracy is really a measure of wet print accuracy. That’s what really matters. For wet print accuracy, the governing specification is to have a value of ±25 µm – this number derives from the following:
  • Machine accuracy = ±12 um
  • Stencil accuracy = ±12 um
  • Total = ±25 um.
Now here is where the argument begins. If we increase the accuracy of our machines and combine this with more accurate stencil patterns, the issue remains as to whether or not the PCB accuracy specs are within ±50 µm or even greater, not including routing and stretch errors. If the tolerances for the pallet that we are printing on is greater than the combined accuracy of the machine and medium we print through, then the PCB remains the wildcard and negates any overall process improvements. Now there’s a thought to stir things up.
Speed, throughput or cycle time?
Throughput is what matters; it’s really the goal. But it is a function of the total cycle time. Printing speed itself doesn’t matter because print speed/squeegee speed is governed by the paste, the size of the board being printed on, aperture sizes, and other factors that must be optimized to maximize yields. Indeed, there is one optimum print speed for every specific application or PCB.
Total cycle time is the sum of all cycle times related to a single PCB, including squeegee speed, board indexing, clamping, etc., vision alignment, and the stencil wipe cycle. And yet, for clarification, the printer has not been the bottleneck in the process for some time now. Still, there continue to be attempts to achieve sub-15 second cycle times and the occasional sub-10 second. Higher speeds have – logically – made Inspection the new challenge on the line, since both SPI (solder paste inspection) and AOI (automated optical inspection) have the biggest jobs in terms of detecting problems before they become part of assembled boards (i.e. for process optimization). Or in the case of AOI, finding process failings before the product is shipped. Issues that can be addressed upstream ahead of time. The rule is to always make adjustments below the stencil, i.e., where squeegee speeds need to remain constant. Squeegee speeds are governed by the paste chemistry, and the variables such as release profile, transport handling, and reduced wiping have the most impact on overall throughput and cycle time. One must also take into consideration dispensing (where needed) and adding of solder paste to the stencil as overhead functions.
Closed-loop communication between the printer and a downstream solder paste inspection is another powerful tool that enhances yields and improves throughput by correcting printer registration errors ‘on the fly’ and thereby minimizing print defects. Closed-loop SPI/printer communication allows controlled feedback of the offset data from the SPI to be applied to the printer. It eliminates the need to change offsets due to material changes as well.
Stencil wiping
Stencil wiping frequency affects throughput and adds to the overall cycle time average per shift. It’s simple math; we can make cycles more efficient and shorter in duration, e.g., using solvent-based stencil wiping in fewer strokes. With a more optimized print process, the frequency of stencil wipes can be lessened, boosting overall throughput. One wipe in 10 prints allows for higher production volumes than one wipe every 3 prints. There is no established industry standard for a stencil cleaning interval. It depends on the complexity of the assembly, the solder paste used, stencil design and printer settings.It’s always advisable to consider the Wiping Rule of Thumb: If your wipe frequency is greater than one in 15 prints, then the gasketing between board and stencil, and aperture design, are working in perfect harmony. If the wipe frequency is less than 10 for “normal” boards, then we can have a kind of mismatch. And yet increased wiping of less than one in 5 is often a characteristic of BGA or fine-pitch-device patterns where stencil area ratios are below or at minimal requirements for this demanding PCBA technology.
Perhaps in a nutshell we can postulate that a PCB design usually has a “sweet spot” where it can run at an optimum speed without creating process defects, or at least creating the minimal number possible. This results from the design for manufacturing (DFM) of PCB design. A maximum speed of a product can be predetermined, i.e., before the first print. Component mix, position, density, multi-up and routing can all determine how fast a board can be run. One more rule of thumb is that if you’re going to run fast, then SPI becomes a requirement. Keep in mind the maxim that “You can build a lot of boards, but you can also build a lot of rework” if an out-of-control process goes on unchecked.
Current Issue
Titelbild EPP EUROPE Electronics Production and Test 11
Issue
11.2023
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